This invention relates to a variable-speed pumped-storage power station, and more particularly to a variable-speed pumped-storage power generating system which is suitable to stably continue pumping operation under command of an output command signal.
A variable-speed pumped-storage power generating system is commonly known from the disclosure of, for example, U.S. Pat. No. 4,481,455. The disclosed system includes a generator motor which has a primary winding connected to an a.c. power system through a main circuit including a breaker and a main transformer. The generator motor is directly coupled at its rotor to a prime mover/load and has a secondary winding connected to the a.c. power system through an excitation circuit including a frequency converter and an exciting transformer. The system disclosed in the cited patent has such a great advantage that the prime mover/load can be driven at a rotation speed independent of the frequency of the a.c. power system. In an application of the power generating system to a variable-speed pumped-storage power station, a turbine/pump is coupled directly to the rotor of the generator motor as the prime mover/load, and, during generating operation, the rotor of the generator motor (the generator) is driven by the water turbine, thereby inducing power in the primary winding of the generator motor. The induced power is supplied to an electric power system. On the other hand, during motoring operation (pumping operation), the generator motor is driven as the motor by the power supplied from the electric power system, and water is pumped up by the pump coupled directly to the rotor of the generator motor. In this case, the frequency of the electric power system is constant or, for example, 60 Hz. However, the rotation speed of the turbine/pump can be freely selected independently of the frequency of the electric power system. Thus, the rotation speed is selected to maximize the operation efficiency of the water turbine or pump.
The fact that the rotation speed N of the turbine/pump can be freely selected means that there is a slip frequency f.sub.2 between the frequency f.sub.1 of the electric power system and the frequency f(=(P.times.N)/120, P: the number of poles of winding) corresponding to the rotation speed N of the turbine/pump, and the following equation (1) holds: EQU f.sub.2 =f.sub.1 -f=f.sub.1 -(P.times.N)/120 (1)
The slip frequency f.sub.2 in the equation (1) is the frequency of the secondary winding of the generator motor, and the rotation speed N can be set at a value which maximizes the pump efficiency or turbine efficiency while maintaining constant the frequency f.sub.1 of the electric power system. There are two principal manipulated variables that can be regulated to satisfy the equation (1). One of them is the opening of inlet valves or guide vanes in a conduit leading to the turbine/pump, and the other is the firing angle of thyristors constituting the frequency converter. These variables are suitably controlled according to a power command signal given to the variable-speed pumped-storage power station.
U.S. Pat. No. 4,481,455 cited above does not refer to a manner of concrete control of such a variable-speed pumped-storage power station. In this connection, a patent application, for example, JP-A-60-90991 (corresponding to U.S. Pat. No. 4,625,125) discloses a control apparatus suitable for controlling such a poswer station although its principal intention is control of the power station during generating operation. According to the disclosure of the known patent application, an output command signal and a turbine's head signal are applied as inputs to function generators which generate an optimum speed command signal and an optimum guide vane opening command signal respectively. A signal representing the actual rotation speed of the water turbine corresponding to the former command signal is fed back to control the firing angle of the thyristors of the frequency converter, and a signal representing the actual opening of the turbine's guide vanes corresponding to the latter command signal is fed back to control the opening of the guide vanes of the water turbine. Further, a frequency variation or power variation in the electric power system is detected to correct the output command signal thereby consequently correcting the opening of the guide vanes. The disclosed control apparatus is intended exclusively for controlling the generating operation of the power station, and the output command signal applied as an input is used merely as an auxiliary signal for obtaining a target speed signal and a target guide-vane opening signal. That is, the output command signal is not directly compared with an actual generator output. Thus, from the viewpoint of output control (of the generator), open loop control systems are provided for the speed control and guide-vane opening control, and the generator output is determined as a result of the above controls. From the viewpoint of the generator output control, such open loop control systems, in which the rotation speed control is a primary object, and the generator output control is a secondary object, do not quickly respond but respond with a delay time. Further, although plural control systems such as the rotation speed control and the generator output control are desired sometimes to be applied to the same generator any practical counter-measure against interference therebetween has not been realized yet. In the nighttime when the load of the electric power system is light, most thermal power plants are stopped and a greater part of the load is supplied from nuclear power stations which must make base load operation, hence, which do not have a frequency regulatability, and the remaining part is borne by hydro-electric power stations and other thermal power stations operable with a low generation cost. That is, for the purpose of an economical use of the electric power system, the thermal power stations requiring a high generation cost are shut down in the nighttime. Thus, shortage of the electrical output and unsatisfactory frequency regulation may occur in the nighttime. That is, the function of regulating the quantity of supplied power according to a load variation and accordingly the function of maintaining the frquency of the power system substantially constant are not fully exhibited.
A pumped-storage power station of variable-speed type provides various merits. One of the merits is that the power station can operate at a high efficiency in each of the generating operation mode and the pumping operation mode. According to another merit, the function of controlling the frequency of the electric power system can be effected even when the power station operates in the pumping mode, that is, when the generator motor operates as the motor. These are most desirable merits in use of the pumped-storage power station of the variable-speed type.
However, the known control system, which is based only on the prmise that the rotation speed is controlled by controlling the output of the motor, is not satisfactory in frequency control of the electric power system when operated in the pumping mode, especially, in terms of response time and compatibility of its power control and speed control.